Moved SimpleWeb::string_view to utility.hpp, and made use of string_view in Server::Response::write

utility.hpp

@@ -8,6 +8,22 @@
 #include <string>
 #include <unordered_map>
 
+#if __cplusplus > 201402L || (defined(_MSC_VER) && _MSC_VER >= 1910)
+#include <string_view>
+namespace SimpleWeb {
+  using string_view = std::string_view;
+}
+#elif !defined(USE_STANDALONE_ASIO)
+#include <boost/utility/string_ref.hpp>
+namespace SimpleWeb {
+  using string_view = boost::string_ref;
+}
+#else
+namespace SimpleWeb {
+  using string_view = const std::string &;
+}
+#endif
+
 namespace SimpleWeb {
   inline bool case_insensitive_equal(const std::string &str1, const std::string &str2) noexcept {
     return str1.size() == str2.size() &&